Power Cord Length


Here's one for the electric wizards to kick around. Whilst attending Axpona this weekend I was doing some power cord shopping. At one particular display I was being told by a very well known cable company representative, I'll not mention the name so it doesn't influence any responses, that the rule of thumb is that in order for the pwr cord to be effective it needs to be at least five feet long. This allows the current to be in the cord long enough for it to be effective. Ok, I'm really dumb when it comes to this sort of thing so maybe I said that in the most simpliest of words but that's pretty much how it was explained. Anyone care to elaborate on this good or bad?
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It must also be said that many designers are not going to give away their particular paradigm or trade secret while others do have white papers and measurements galore it is a complex issue. Ultimately it is your own listening judgement and I urge you to try experimenting. The relatively inexpensive DH Labs bulk DIY cable is a great place to start.


No, it's not a complex issue.  A power cord is a very simple issue. Building an actual power amplifier IS a complex issue.  Power cords, speaker cables, etc. are not complex at all.

What is really complex is our brains and the physiology of our acoustic system and perception.  Every time I have a couple of beers my system sounds so much better. 
I did research and bought a spool of Belden 19364 wire. This wire is 14ga, shielded with a drain. I bought good plugs for the ends. I did research how long the wire needed to be to not let the cycle length of the electricity exit the cords and pollute the rest of the system. I think it was 4' or so, I did this a year or so ago. I plugged them into my monoblocks for a listen and was very presently surprised. Before this I had used computer power cords that I had on hand. I also use a power conditioner, so I changed out that cord a week or so later and heard no difference. I've played around with different interconnects and speaker wires as well. Some wire leaves me flat others make life grand. If you are a person that can hear no difference it is you or your gear. Top end equipment that has the ability to provide your ears with frequency beyond your hearing limits provides subliminal spacial imaging as well with proper harmonizing cables and cords.
@dconrad,
You responded to an old thread, but, it is good that you tried upgrading your power cables. These things can make an 'improvement', but, as you have found out, it depends upon the equipment associated.
FWIW, I seem to have settled on Audioquest for my wiring.
B
The reason minimum length is stipulated with some power cords is to have enough resistance in the cord to drop noise across it. Understand how a voltage divider works? For argurment’s sake, let’s say the RF choke of your power supply input stage has a reactive impedance of 10 ohms at 100khz and your power cord has a resistance of 10 ohms also. The RFI noise will be equally divided, 50% across the cord’s resistance and 50% across the RF choke on the power supply input. If we make that power cord much shorter, say half as long, we reduce its resistance to 5 ohms while the RF choke reactive impedance stays the same at 10 ohms. V(choke)=V(in)*[Z(choke)/Z(choke)+R(powercord)] so calculating for 100volts RFI, V(choke)=100*(10/15). So V(choke)=66.7 volts, or put another way, 66.7% of the RFI noise is dropped across the input of the power supply, where latter filtering stages need to deal with it. 

With that said, we only need to find a suitable amount of ballast resistance in the power cord and branch circuit to dissipate as much RFI energy as we can. A common SJT extension cord made of the minimum AWG to bear the current demands of the device is all you really need. A heavier than necessary cord will exhibit less resistance that is an integral and essential part of the resistor/inductor filter. For most low to midrange power amplifiers, an 18AWG power cord will be your best overall choice. 18AWG is also the best choice for low-power consumption devices such as preamps and CD players. 

Sure, a big fat 12AWG power cord looks impressive to the uninitiated, but if you understand the principle I outlined above, and your gear was endowed by its manufacturers with decent power supply input filtering, you now will understand that all that glitters is not gold in the world of power cords.